Composite materials have matured over the years since the inception of glass reinforced thermoset materials to the advent of thermoplastic reinforced material. Thermoplastic materials can provide unique advantages over thermoset materials. Thermoplastics do not require a chemical reaction to form the structural part. For example, in the formation of thermoplastic materials the undesirable release of Volatile Organic Compounds (VOCs) associated forming thermoset materials can be avoided. However, composite or reinforced thermoplastic components cannot simply be formed using common injection molding or rotomolding techniques due to the presence of the reinforcing materials. As discussed below, efforts have been made to heat the molds in order to in turn heat the thermoplastic materials to permit them to be shaped to the mold. The heating of the molds themselves and, in particular, metal tooling, can require a high degree of heat to be applied as well as a large amount of time for the tooling to be brought up to the desired temperature. The result is that such techniques can be costly, requiring a high degree of energy and a long cycle time, and result in increased cost for the components.
Much development has gone into the impregnation of a thermoplastic in combination with a fiberglass or other reinforcing fiber wherein a material can be made that achieves similar strength and flex modules to traditional fiber reinforced thermoset products. Such materials can be used in numerous applications, for example, from aviation to race cars to “bullet proof” materials. Numerous manufacturers have developed a combination of plastic and or other reinforcing articles that can than be formed or shaped into the final product. One such thermoformed material is described in U.S. Application Ser. No. 61/151,811, filed Feb. 11, 2009, incorporated herein by reference in its entirety. Another such material is available from Owens Corning, a manufacturer who produces a thermoformed product commonly known within the industry as TWINTEX. These materials can be provided in numerous configurations and densities. One type would be a comingled roving which may be woven in to a desired pattern or cloth type. Alternatively, the comingled roving may be provided in a random pattern. Another type of thermoplastic reinforced material, such as POLYSTRAND, is one where the plastic is laminated to the fibers and where the reinforcement is co-laminated as the thermoplastic is extruded to provide for a sheet-type material which can than be formed into a final product.
While these materials can be useful in certain applications, challenges and limitations exist due to difficulties encountered when trying to form or mold such materials, including using the above-described techniques as well as those described below.
One such method is filament winding. This is a traditional method used by manufacturers who utilizing a reinforced polymer with a long-strand fiber typically an E- or S-type fiberglass intermingled with the thermoplastic which is then sent through an oven where the material is heated beyond the forming temperature of the material then it is wound around a mandrel and cooled to the desired temperature.
Another method is known as thermoforming stamping and press molding. For example, this method is traditionally used with the fiber and thermoplastic has been consolidated into a sheet for such as is typical with a POLYSTRAND type material provided by Polystrand, Inc. of Montrose, Colo. The desired amount of consolidated sheets are stacked and clamped into a carrier device which than goes into an oven that would typically use infrared or other types of heating methods to bring the thermoplastics within the structure up to their forming temperature. The material is then moved into the forming station where it can be formed by the use of vacuum. In another form of this method, a top and bottom press or platens are used to physically push the material into the desired form which is then cooled. Core materials may be incorporated using this type of forming. These cores can be used to create a sandwich structure.
Vacuum molding is another method that may be used and typically is used when forming larger more complex shapes. The thermoplastic material is typically woven into a mat form and positioned onto a mold. The material is selected and/or layered in such a manner as to achieve the desired thicknesses and properties. A vacuum bag is placed over the material and mold and sealed to the surface. A vacuum is then applied after which the entire mold is heated beyond the forming temperature to shape the material. The mold is then cooled below the forming temperature to allow the thermoplastic material to cool back to a solid state.
Another method of forming thermoplastic materials is direct compression and injection. In this method, the reinforcing material (e.g., glass carbon fiber or other such type material) is introduced at the head of an extruder which is extruding the thermoplastic which is to be reinforced. The glass or roving material is in a chopped form and is introduced into the screw creating a hot molding compound consisting of the plastic and the reinforcement and it can either be quickly transferred to a press for compression molding or into what is typically called a shot pot where the material can be introduced into an injection type molding machine very common within the industry. Similar to direct compression and injection, in co-molding the fibers are introduced to the extruder but in this case are introduced to the thermoplastic material outside the screw typically in a mat or woven format where the two are consolidated typically into a sheet material for further processing.
Yet another method is traditional injection molding. In this process the reinforcing fibers are introduced to the pellet form of the thermoplastic and than introduced into a typical screw where the two comingle and than are injected into a mold to form the final part. One further example method is diaphragm forming. In this method, two sheets of silicon are used as a carrier to the reinforced thermoplastic. The material is positioned between two sheets of silicon, and placed between two hot platens to introduce heat to increase the temperature of the material above the forming temperature. The material is then moved to a forming station where it can be formed using positive air pressure. The silicone sheets may then be removed once the entire sandwich has cooled sufficiently. An additional prior method included vacuum bagging a thermoplastic piece to a one-sided tool and placed in an oven or autoclave to heat to the desired temperature for forming. Alternatively the tooling is heated to heat the material. Then tool would then be cooled and the molded part removed from the tool.
One disadvantage to the aforementioned methods of forming this type of material is that it can be time consuming to heat the material, move it into a molding station, form the material and then cool the material. This is in part due to the time required to needed to cool the material formation of each molding method. Another disadvantage is that the tooling requirements can be very expensive due to the need or ability of each mold to be heated and cooled in a direction from the exterior toward the part, which can thereby require aluminum or steel type molds which are very expensive to produce and difficult to maintain. For example, extensive fluid flow paths may be formed in the tooling. Another disadvantage to these types of methods of forming reinforced thermoplastic parts is the cycle time required to complete each piece in comparison to traditional thermoset materials. Thermoset materials use a chemical reaction to form the solid which can take a relatively short amount of time, but as discussed above, can be accompanied by the release of VOCs.
Thermoplastic materials are by-products of the petroleum industry and have a much lower cost of raw materials. However, historically the high cost of forming each piece of the thermoplastic material due to heating and cooling times of the molds and the associated cost of heating the molds renders thermoplastic pieces expensive in comparison to thermoset pieces. Therefore, in order to make thermoplastic products competitive, there must be a decrease in the cycle times and energy costs to reduce the overall cost of the end product. In addition thermoplastic material may be selected for the unique properties contained.
Much development has gone in to the molds that are used in the forming of thermoplastic materials. Examples of methods and apparatuses that may be used in the forming of thermoplastic materials are disclosed in U.S. Patent Ser. No. 61/261,193, which is incorporated herein in its entirety. For example, a heated bladder may be placed in contact with the thermoplastic material to force the thermoplastic material into contact with a mold. In another example, the mold itself may be heated to raise the temperature of the thermoplastic material above its forming temperature. A vacuum may also be used to pull the thermoplastic material into contact with the mold during the shaping process.
Thermoplastic materials may be provided in shaped forms that are produced from a single sheet of thermoplastic material. For added strength and/or thickness, multiple sheets of thermoplastic material may be layered and thermoformed together. By applying heat to the thermoplastic material, the sheets melt together and combine to form a single formed unit with increased thickness and/or strength as compared to a single sheet of material. In another embodiment, cores may be placed between layers of thermoplastic material to provide additional strength and stiffness to the thermoplastic material. The use of typical cores, such as foams and honeycombs, to form composite materials can limit the processes and resulting shapes. For instance, most all composite sandwiches are flat or only curved in one dimension. This is because the combination of materials can be very difficult to lay up and form into complex shapes.